# Health (readiness / liveness / active-node)

Second normalized component under the operability cluster. **Goal: path to shared code** — converge
the three sister projects onto a common three-tier health endpoint convention and a set of shared
probe implementations, proposed as the `ZB.MOM.WW.Health` library set (3 packages), while each
project keeps its own probe registration and orchestrator wiring.

- The one target: [`spec/SPEC.md`](spec/SPEC.md)
- The proposed shared library: [`shared-contract/ZB.MOM.WW.Health.md`](shared-contract/ZB.MOM.WW.Health.md)
- Divergences + backlog: [`GAPS.md`](GAPS.md)
- Current state, per project: [`current-state/`](current-state/)

## Why health is a strong normalization candidate

Both OtOpcUa and ScadaBridge trace their health-check structure to the same "ScadaLink three-tier
pattern" (`HealthEndpoints.cs:13` says so explicitly) but have already diverged in probe logic,
status semantics, response writer, and endpoint registration style. MxAccessGateway has no shared
ancestry here — it has a single hardcoded `/health/live` endpoint with no real probes at all.
The common core (three tiers, database probe, Akka cluster probe, active-node probe) is
re-implemented twice and absent once. Shared probe implementations with configurable policies
close the gap without forcing identical behavior onto projects with legitimately different cluster
semantics.

## Status by project

| Project | Endpoints today | Probes today | Response writer | `/healthz` | `IActiveNodeGate` | Adoption status |
|---|---|---|---|---|---|---|
| **OtOpcUa** | `/health/ready`, `/health/active`, `/healthz` | Database (query), AkkaCluster (2-way), AdminRoleLeader (role-filtered) | Default (plain-text/JSON) | ✅ present | — | Not started |
| **MxAccessGateway** | `/health/live` only (raw `MapGet`; hardcoded `"Healthy"`) | **None** (`AddHealthChecks()` called but unused) | Bespoke `GatewayHealthReply` JSON | ⛔ absent | — | Not started |
| **ScadaBridge** | `/health/ready`, `/health/active` | Database (`CanConnectAsync`), AkkaCluster (3-way), ActiveNode (role-less) | `HealthChecks.UI.Client` JSON | ⛔ absent | `ActiveNodeGate` (backs Inbound API 503 gate) | Not started |

See each project's [`current-state/<project>/CURRENT-STATE.md`](current-state/) for the
code-verified detail and its adoption plan.

## Normalized vs. left per-project

**Normalized (the shared target):**

- Three-tier endpoint convention: `/health/ready` (tag `ready`), `/health/active` (tag `active`),
  `/healthz` (bare liveness). Mapped by `app.MapZbHealth()` from `ZB.MOM.WW.Health`.
- Canonical JSON response writer (lifted from `HealthChecks.UI.Client` style; no per-project
  writer wiring needed).
- `IActiveNodeGate` seam — generalized from ScadaBridge's `ActiveNodeGate`; wired into `MapZbHealth`
  for automatic active-tier response.
- `GrpcDependencyHealthCheck` — reachability probe for a downstream gRPC dependency (covers
  OtOpcUa → MxAccessGateway channel and MxAccessGateway → worker IPC).
- `AkkaClusterHealthCheck` (in `ZB.MOM.WW.Health.Akka`) with a configurable status policy.
  Default = ScadaBridge's three-way policy; `OtOpcUaCompat` preset preserves OtOpcUa's two-way
  self-Up-among-members scan.
- `ActiveNodeHealthCheck` (in `ZB.MOM.WW.Health.Akka`) with an optional role filter. Role-less =
  ScadaBridge's behavior (Up + cluster leader); role-filtered = OtOpcUa's `AdminRoleLeader`
  behavior.
- `DatabaseHealthCheck<TContext>` (in `ZB.MOM.WW.Health.EntityFrameworkCore`) with default
  `CanConnectAsync` and an optional `ProbeQuery` delegate.
- `AllowAnonymous` on all three tiers by default (consistent across all three projects today).

**Left per-project (not forced together):**

- Which probes each app registers, their names, and which tags they carry.
- Orchestrator / Traefik wiring (sidecars, route rules, upstreams).
- ScadaBridge's `HealthMonitoring/` distributed aggregation pipeline (`SiteHealthCollector`,
  `CentralHealthAggregator`, `HealthReportSender`, etc.) — domain-specific, no shared-library
  equivalent.
- MxAccessGateway's `GatewayHealthReply` metadata (`DefaultBackend`, `WorkerProtocolVersion`) —
  keep as a bespoke `/info` endpoint.
- The x86 worker process — out of process and out of scope; the gateway-side
  `GrpcDependencyHealthCheck` observes it indirectly.

## Package structure

`ZB.MOM.WW.Health` ships as three dependency-split packages:

| Package | Contents | Consumers |
|---|---|---|
| `ZB.MOM.WW.Health` | Core tiers, `MapZbHealth`, canonical writer, `IActiveNodeGate`, `GrpcDependencyHealthCheck` | All three |
| `ZB.MOM.WW.Health.Akka` | `AkkaClusterHealthCheck` + status presets, `ActiveNodeHealthCheck` + role filter | OtOpcUa, ScadaBridge |
| `ZB.MOM.WW.Health.EntityFrameworkCore` | `DatabaseHealthCheck<TContext>` + optional probe delegate | OtOpcUa, ScadaBridge |

MxAccessGateway consumes the core package only (no Akka, no EF). OtOpcUa and ScadaBridge consume
all three.

## Component status

**Status: Draft — library built at 0.1.0.** Spec and shared-contract written; current-state docs
verified; GAPS backlog populated. Library implemented and packed at
[`../../ZB.MOM.WW.Health/`](../../ZB.MOM.WW.Health/) (3 packages, 58 tests;
`ZB.MOM.WW.Health`, `ZB.MOM.WW.Health.Akka`, `ZB.MOM.WW.Health.EntityFrameworkCore`).
Adoption by the three apps is the next follow-on tracked in [`GAPS.md`](GAPS.md).
